Abstract: The present disclosure relates to mutant polypeptides derived from Escherichia coli dihydrofolate reductase (DHFR) which can be fused to a polypeptide of interest for efficient conditional modulation of its activity. Also disclosed are polynucleotides encoding such mutant polypeptides, vectors comprising such polynucleotides, and the use of such polypeptides, polynucleotides and vectors for treating a disorder.

Abstract: The present invention relates to a novel gene expression system comprising: a) a first nucleotide sequence encoding a fusion polypeptide of: a1) a destabilizing domain (DD) based on DHFR, and a2) a GTPcyclohydrolase 1 (GCH1) polypeptide, or a biologically active fragment or variant thereof; and b) a second nucleotide sequence encoding a tyrosine hydroxylase (TH) polypeptide, or a biologically active fragment or variant thereof. The invention also relates to use of this gene expression system together with a ligand binding to a destabilizing domain (DD) based on dihydrofolate reductase (DHFR) for treatment of diseases associated with a reduced dopamine level, such as Parkinson's disease.